Jeep Strokers

I understand the concept between the short and long rod stroker.

However, when you throwing a 4.2L crank to build the stroker, we end up with a .482 longer stroke.

Now the difference between the 4.2 rods and 4.0 rods is .25... with the 4.2L rods being the shorter ones.

Now if we do the math using the 4.2L rods, we are still using an increased stroke of .232 compared to stock..

So my question is this.. how is it any different running 4.2L rods in a stroker than running 4.0 rods stock. You still have a technically longer stroke than a stock 4.0, so how could longevity and reliablility be compromised??

Make since? Or am i looking at this backwards, sideways and upside down.

you have the same stroke with both rods. The longer rods put less side load on the pistons because they are longer and don't make as sharp an angle as the crank rotates. That is the only thing that can affect longevity as far as I know. Also the long rods give better quench height. I have also heard that the long rods will have a longer dwell time at the bottom of the stroke allowing more fuel/air to enter the chamber.

Also, the piston skirt is pulled out of the bore less with the long rods

RAPTORFAN85 wrote:I have also heard that the long rods will have a longer dwell time at the bottom of the stroke allowing more fuel/air to enter the chamber.

Actually it's the opposite. Short rods stay at the bottom of the cylinder for longer time, long rods spend more time on the top of the cylinder. This means, that short rods are better for high revs applications, where the whole cycle is very short and every millisecond of intake time counts. But longer rods permit for better chamber scavenging during exhaust, and also give the mixture more time to burn completely under high pressure. Theoretically, at given compression, the average cylinder pressure will be higher on long rod stroker.

Also, when you decide to use stock ecu programming, the spark advance is more beneficial in long rod stroker. Again, piston then is already high enough, so the spark gives better effect. Also, the quench turbulence starts earlier (it's a tiny bit weaker though, so less quench is needed for the same turbulence effect).

Another important factor is that our long rods are a little lighter and are considered a little stronger (better casting)

Generally, for most applications here the stock long rods are better than stock short ones.

mendelmax wrote: Actually it's the opposite. Short rods stay at the bottom of the cylinder for longer time, long rods spend more time on the top of the cylinder. This means, that short rods are better for high revs applications, where the whole cycle is very short and every millisecond of intake time counts. But longer rods permit for better chamber scavenging during exhaust, and also give the mixture more time to burn completely under high pressure. Theoretically, at given compression, the average cylinder pressure will be higher on long rod stroker.

That doesn't seem right either, The time spent at the top of the cylinder and the time spent at the bottom should be the same for any given rod. So how can one be longer at the top and one at the bottom? doesn't seem logical...

It's hard for me to explain it verbally, but it will help to draw it.

Imagine that the rod length is equal to the half of the stroke (radius). This would mean, that despite the crank rotation, the hypothetical piston would stay in the same position all the time. Now imagine it being longer, stroke-length for example. Then some of the length is taken for side movement of the large rod end. The longer the rod is, the less percent of it's travel is taken for the side travel, and more for up-down movement.

If you don't believe, go into the CR calculator, enter some IVC angle and check, how compression ratio will change with the change of rod length. With shorter rods the CR will be higher, meaning that at given crank rotation degrees, the piston has moved up less.

mendelmax wrote:It's hard for me to explain it verbally, but it will help to draw it.

Imagine that the rod length is equal to the half of the stroke (radius). This would mean, that despite the crank rotation, the hypothetical piston would stay in the same position all the time. Now imagine it being longer, stroke-length for example. Then some of the length is taken for side movement of the large rod end. The longer the rod is, the less percent of it's travel is taken for the side travel, and more for up-down movement.

If you don't believe, go into the CR calculator, enter some IVC angle and check, how compression ratio will change with the change of rod length. With shorter rods the CR will be higher, meaning that at given crank rotation degrees, the piston has moved up less.

Reread what you have wrote. It makes no sense.

The CR calculator is WRONG.

The crank pin has to swing through the same degrees of arc whether it is at BDC or TDC.
The longer rod will give a longer dwell time at both BDC and TDC. In addition the longer rod will increase piston acceleration as the rod as the angle draws towards zero. Note I did not say RPM but piston acceleration. However, overall piston speed with be the same between the S/L rods. There is much more to be gained by comparing bore to stroke ratio`s then concentrating on rod length.

Taking the factor of deck height/quench out of the OPers question. The longer rod will:

Increase dwell at both TDC and BDC
Increase piston acceleration
The longer rod will inecrease rod angularityl

RAPTORFAN85 wrote:you have the same stroke with both rods. The longer rods put less side load on the pistons because they are longer and don't make as sharp an angle as the crank rotates. That is the only thing that can affect longevity as far as I know. Also the long rods give better quench height. [b]I have also heard [/b]that the long rods will have a longer dwell time at the bottom of the stroke allowing more fuel/air to enter the chamber.

Sorry buddy but much of your knowledge seems to be coming from this source.

jsawduste wrote: Sorry buddy but much of your knowledge seems to be coming from this source.

I was right wasn't I? Bad wording I guess.

jsawduste wrote:The CR calculator is WRONG.

How is it wrong?


Back on the subject though .. is 1/4" (.25) really that much when it comes to rod length?

jsawduste wrote:Reread what you have wrote. It makes no sense.

The CR calculator is WRONG.

I don't want to argue, but just draw it and you'll see what I mean.

If you are right, then all the CR calculators are wrong. Why bother entering rod length if it doesn't matter?

Muad'Dib wrote:Back on the subject though .. is 1/4" (.25) really that much when it comes to rod length?

Well, probably not that much, but the differences have to be considered. Also it's not just about the length in this case, but also weight and strength. I did my last stroker on short rods for budget reasons, but the next one will be long-rod for sure.

How long did your last stroker run (or still running)? Miles?

Muad'Dib wrote:How long did your last stroker run (or still running)? Miles?

It was built for my buddy, I don't know how many miles he has now, probably around 2000-3000. It's pretty new engine.

And I don't want to replace it, this time I'll be building mine

I spent some time doing a simple drawing. I regret I used so small scale, since it's not as accurate as I would like it to be, but it shows my point. To make things clearly visible I used 60mm stroke and two extremely different rods: 70mm and 140 mm. I made drawings for 45, 90 and 135*, if someone is more ambitious I encourage to draw it larger with more angles. It clearly shows why longer rod leaves the bottom part of the cylinder earlier and stays at the top longer. Sorry for poor scan, the parts drawn with pencil are not too well visible, but everything is clear.


This drawing also shows why cylinder walls are wearing less in long-rod stroker- the angle between vertical axle and actual rod position is smaller, thus piston is supported more on the rod, less on cylinder wall. This wall pressure is a wasted energy, which also robs power from our engine. Not much, but little differences added together may make a real power.

To me the biggest drawback of using long rods seems to be the high ring position on the piston. They get close to the top, which in the long run may lead to seizing from overheating. Not a big deal on low duty applications, but for someone who wants to floor it often, use NOS or turbo this gets important. It can be somewhat solved by using ceramic piston coat. We did it on our last stroker and we hope it will help somewhat with excessive heat caused by propane (burns hotter than gasoline).

Your math is flawed. Simply using a 70 and then a 140mm stroke should indeed give you twice the values. As 70 x 2 = 140

But that is not the question.

In order for the piston to remain at a constant (set) deck height, be it BDC or TDC the pin height in the piston must change the same as the difference in rod length. Such is the case of 944`s. They have a different pin height then a stock piston. Which moves then up in the bore. THE SAME AMOUNT AT TDC AS AT BDC.

Also the crank has the same rotational distance to follow as it transitions from an ascending to a descending travel. AND THE SAME AMOUNT OF DWELL TIME AT TDC AND BDC.


Another example;
I recall building long rod 413 Pontiac's in the 70`s. +.030 455 piston in a .060 overboarded 400 sized bore with a zero deck height. The difference was pin height. Both blocks share the same deck height. The difference was the longer rod to make up for the increased pin height.

So given the fact that the stroke is determined by the crank offset. The piston will travel travel the same amount. Any change in rod length must correspond with a change in piston pin height. IF the same deck heights are used.

Now think about that. Does the piston hang off the top of the rod ? Or is the rod buried deep within the piston, close to the crown ?

As I said, I won't argue, because you don't get my point, and I can't explain it more. Just look at the drawing I made. The stroke is the same, only rod lengths are 70 and 140. Don't mix these two things.

So yes, the stroke is the same, yes, the overall piston travel is the same, and yes, the dish height at TDC and BDC are in exactly the same places if the pin is moved to compensate for rod length, but the movement of the piston is different.

When moving at compression stroke the piston accelerates faster with long rod, and decelerates slower. Therefore, at given crank rotation degrees, the piston is higher at the bore, especially below 90*. This means, that both pistons start at the same point at BDC, then the long-rod one goes faster up the bore, and then the difference gets smaller and smaller with rotation angle increase, and after 90* they are getting close to each other, meeting exactly at TDC. It's all explained on the drawing.